Semiconductor device performance is related to carrier mobility. For example, in metal-oxide semiconductor (MOS) devices, the higher the carrier mobility in the channel of the device, the faster the current can flow in the channel and the faster the device can perform.
Carrier mobility is determined by properties of the semiconductor material such as its lattice constant. Stress in the semiconductor substrate can change the lattice constant and thus the carrier mobility. One way to add stress to the substrate is to add a contact etch stop layer. Further, the thickness of the spacers on the sidewalls of the gate electrode also affect the effect of stress on the substrate. Thick spacers reduce the desirable impact of the strained contact etch stop. However, thick spacers are desirable during the manufacture of semiconductor devices to control short channel effects. Therefore, there is an inherent conflict on the spacer thickness to achieve a well-performing semiconductor device.